1. Field of the Invention
The present invention generally relates to an air conditioner apparatus and, more particularly, to an air conditioner apparatus in which a refrigeration cycle is formed by coupling an outdoor unit having at least two compressors to at least one indoor unit.
2. Description of the Related Art
A conventional multi-type air conditioner apparatus is constituted by coupling an outdoor unit and a plurality of indoor units through a branch unit, as shown in FIG. 5 (prior art).
Referring to FIG. 5 (prior art), reference symbol A denotes an outdoor unit including a plurality of compressors, e.g., one (variable-capacity) compressor A1 driven at a variable frequency and a (fixed-capacity) compressor A2 driven by a commercial power source. The outdoor unit A is connected to a branch unit B. The branch unit B is connected to a plurality of indoor units C.sub.1, C.sub.2, and C.sub.3.
The indoor units C.sub.1, C.sub.2, and C.sub.3 respectively send required capacities corresponding to the respective air-conditioning loads to the branch unit B as frequency setting signals f.sub.1, f.sub.2, and f.sub.3.
The branch unit B obtains the required capacities of the indoor units C.sub.1, C.sub.2, and C.sub.3 from the sent frequency setting signals f.sub.1, f.sub.2, and f.sub.3, and sends a frequency setting signal f.sub.0 corresponding to the sum of the required capacities to the outdoor unit A.
The outdoor unit A controls the number of the compressors to be driven and their operating frequencies in accordance with the frequency setting signal f.sub.0. When the number of the compressors to be driven is to be controlled, the order of priority is set such that the variable-capacity compressor A1 is started first, and the fixed-capacity compressor A2 is started next.
When a heating operation is to be started, the outdoor unit A operates the two compressors A1 and A2 so as to quickly perform hot starting of the heating operation. In addition, the outdoor unit A also operates the two compressors A1 and A2 during a defrosting operation in order to improve defrosting performance.
When a single-compressor operation using the compressor A1 is to be shifted to a double-compressor operation using the two compressors A1 and A2, the operating frequency of the variable-capacity compressor A1 which has already been driven is temporarily decreased, in consideration of oil equalization between the compressors A1 and A2, as shown in FIG. 6 (prior art). In this case, the change speed of the operating frequency is set to be a normal change speed VS1.
In the above-described conventional air conditioner apparatus, when a heating operation is started, a double-compressor operation using the two compressors is performed regardless of a loading state. For this reason, when at least one of the indoor units and outdoor units is in an overload state, the high-pressure-side pressure of the refrigeration cycle is abnormally increased, and a high-pressure protection release is activated to stop all the operations.
In order to prevent this, some apparatuses are designed to suppress an abnormal increase in high-pressure-side pressure by stopping the operation of the second compressor immediately when the high-pressure-side pressure reaches a preset value.
In this case, however, the second compressor is frequently turned on and off, and hence its service life is adversely affected by this repetitive ON/OFF operation.
In addition, when a single-compressor operation is to be shifted to a double-compressor operation, the operating frequency of the variable-capacity compressor is temporarily decreased. However, during this period, a capacity shortage causes discomfort of a user.